The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for preventing the belt from slipping on the pulleys of the belt drive transmission and for preventing shock at the recovery of belt gripping.
A control system is known for a continuously variable belt-drive transmission comprising an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The control system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the engine speed exceeds a predetermined value, an electromagnetic clutch engages, so that the vehicle is started. When the vehicle speed and engine speed exceed set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced, even if the engine speed is kept constant. Thus, the vehicle speed increases with a reduction of the transmission ratio.
In a system for electronically controlling the transmission ratio, a throttle position and driven pulley speed or vehicle speed are detected by sensors to obtain a desired transmission ratio id. The difference id-i between the desired transmission ratio id and the actual transmission ratio i is calculated in order to determine the quantity of oil to be supplied to a cylinder of the servo device of the drive pulley for changing the transmission ratio to the desired transmission ratio. Thus, a primary pressure which is applied to a hydraulic cylinder of the drive pulley is controlled so that the actual transmission ratio i coincides with the desired transmission ratio id.
When the brake pedal is depressed, the speed of the wheels of the vehicle is reduced as well as the body of the vehicle because the wheels grip the road. The primary pressure also decreases with a reduction of the wheel speed so that the running diameter of the belt on the driven pulley is increased to downshift the transmission in dependency on a predetermined change speed pattern.
However, the wheels may be locked when the brake pedal is quickly depressed on a road having a low friction coefficient such as a snowy or icy road. Such a phenomenon is explained with reference to FIGS. 5a to 5d.
When the wheels are locked, the wheel speed Vw rapidly decreases, while the vehicle speed Vm does not quickly decrease because of the slipping of the wheels on the road (FIG. 5c). As a result of a reduction of the speed of the driven pulley, the desired transmission ratio id is increased to downshift the transmission, thereby rapidly decreasing the primary pressure Pp to zero as shown by a solid line in FIG. 5a. Since a vehicle speed sensor is provided which detects the speed of an output shaft of the transmission, the vehicle speed sensor produces a signal which means the vehicle speed largely decreases. As a result, the electromagnetic clutch is disengaged.
Since the wheels are quickly locked, the belt and pulley device are stopped before the transmission ratio reaches a maximum value. Consequently, the belt is held by the gripping operation of the pulleys at a position where the pulleys stop, providing a small transmission ratio i-1 as shown by a dotted line in FIG. 5b.
When the driver notices the wheel lock and releases the brake pedal, the wheels are unlocked so that the wheel speed is quickly increased to coincide proportionally with the vehicle speed Vm. Therefore, the driven pulley is driven by the wheels to rotate the drive pulley through the belt. When the pulleys and belt start to rotate, the gripping force reduces, so that the belt and pulley device are quickly shifted to the large transmission ratio position so as that the actual transmission ratio converges to the desired transmission ratio as shown by a dotted line i-2 in FIG. 5b. Since the inertial mass of a driven member of the electromagnetic clutch, the drive pulley and other members provided between the clutch and the drive pulley is large, a large primary pressure is necessary to grip the belt so as to rotate the drive pulley, the driven member and other members. However, since the primary pressure Pp is still low, the belt slips on the drive pulley.
Since the primary pressure Pp rapidly increases as shown by the solid line in FIG. 5a, the drive pulley suddenly grips the belt, thereby causing a transmitting power change shock. The slipping of the belt and the shock cause deterioration of the belt and hence poor driveability of the vehicle.
Japanese Patent Application Laid-Open 61-290269 discloses a system for controlling the transmission. In the system, a planetary gear device is provided at an output of a continuously variable transmission for controlling the connection of the power train, and for providing a reverse power train. At the sudden braking, the planetary gear device is operated to disconnect the continuously variable transmission from the wheels of the vehicle and to hold the transmission ratio maximum.
However, the disconnection system can only be applied to a planetary gear device provided at the output of the continuously variable transmission and hence it can not be applied to a planetary gear device provided at an input thereof. Moreover, when the brake pedal is released after sudden braking, the continuously variable transmission is connected to the wheels by the planetary gear device. At that time, a great engine braking is effected on the vehicle because the transmission ratio is held at maximum, thereby impairing smooth driving and exerting shock on the belt of the transmission.